I'm new to Veeam ONE, please excuse any ignorance here. I'm also relatively new to Hyper-V, so my terminology may be off since I've spent most of my time in ESXi.

I am seeing alarms about Guest Disk Space on two virtual machines (Hyper-V) in my environment that indicate they have X amount of space left (which is below our threshold). However, when I login to the guests, this is not true, I actually see Y amount of space (which is above our threshold). When I resolve the alarm, it returns almost immediately.

The disk on one of the VMs in question is 249GB total, and from within Windows I can see there is 42.0GB free (~16.8% free).

So - I'm thinking this is actually reporting the size of the VHDX file (which is thin provisioned with a max size of 250GB). However, when I inspect the properties of the disk it shows 239.79GB / 250GB so that doesn't add up to the 23.47GB number report in Veeam ONE either - in fact by that logic it should be reporting even less free space available.

Am I missing something here? Am I misunderstanding the data that the report is reporting on? Or, perhaps, am I misunderstanding how Hyper-V calculates the size of the disk on a guest VM?

Hi Brandon and welcome to the community!
Have you checked what VM disks values are shown under Summary tab?
You can also contact Veeam support to make sure that Veeam ONE collects data from Hyper-V correctly.